The present invention relates to waveguides, antennas and similar devices, and more particularly to a beam waveguide including a pair of dual offset reflector sets that satisfy the Mizuguchi condition and that may be associated with an antenna to send and receive signals.
Satellite systems often require a high gain antenna such as a reflector antenna with a large aperture size to provide high data rate communications either between the satellite and a fixed location on the earth, such as a ground station, or between the satellite and a mobile user with a small, low gain terminal. Realizing such high gain antennas is often a complex interaction between competing needs associated with the spacecraft. For example, blockages by solar panels and other structures associated with the spacecraft, or other antennas should be avoided while mass and complexity are also minimized. In addition, the payload for the high gain antenna may require high power and low losses on the signal path to the aperture of the antenna. One approach is to put the payload for the antenna into a pallet immediately behind the antenna and deploy the entire antenna/payload assembly away from the spacecraft. However, the palletized system may present a large increase in mass and complexity because of the need for separate thermal control and shielding for the pallet and the spacecraft bus. Additional pallet complexity arises due to the need to transmit signals to and from the pallet at some intermediate frequency (IF) if there is a substantial distance between the spacecraft and the pallet. Another issue may be increased complexity in controlling the spacecraft attitude when large masses are moved in a palletized system.
Another approach may be to use a beam waveguide similar that illustrated in FIGS. 1A, 1B, 2A and 2B, respectively. FIGS. 1A and 1B are an illustration of a prior art antenna system 100 including a moveable beam waveguide structure 102 and antenna assembly 104. FIGS. 1A and 1B illustrate the antenna assembly 104 in different rotational positions. As illustrated in FIG. 1B a portion of the structure interferes with a complete range of motion or field of regard of the antenna assembly 104. FIG. 2A is an illustration of a prior art antenna system 200 including a beam waveguide 202 including a set of offset paraboloid reflectors 204 and 206. The beam waveguide 202 may be the same as the waveguide 102 of FIGS. 1A and 1B. FIG. 2B is an adaptation of the prior art antenna system 200 of FIG. 2A illustrating the set of offset paraboloid reflectors 204 and 206 rotated relative to one another as described below.
Some satellite systems require a high gain antenna with a wide angular range of motion or field of regard. In these systems, conventional beam waveguides may be used to enhance the stability of the spacecraft as the antenna moves and to reduce the overall mass of the spacecraft, but achieving a substantially complete field of regard may be difficult due to several factors. Conventional beam waveguides typically have two axes of rotation. These axes are rotated using what may be referred to as an inner gimbal 106 and an outer gimbal 108 (FIGS. 1A and 1B). The outer gimbal 108 may be rigidly tied to the bus of the spacecraft and the inner gimbal 106 may ride the structure that is rotated by the outer gimbal 108. When the inner gimbal 106 rotates such that the main beam of the antenna is nearly parallel to the axis of the outer gimbal 108, the torque required to meet the scan velocity requirements is very high, resulting in regions in the field of regard that cannot be addressed by the antenna. This region of the field of regard may be referred to as the “keyhole.” Another factor is that conventional beam waveguides such as that shown in FIGS. 1A and 1B have a rigid structure that holds two parabolic mirrors, similar to parabolic mirrors 208 and 210 in FIGS. 2A and 2B. As described in more detail below, to avoid distortions and loss of antenna efficiency and power, no rotations should occur between these mirrors. Therefore, the beam waveguide 202 is typically only rotated around mirror axes 212 and 214 in FIGS. 2A and 2B to minimize losses and to reduce the overall mass that is moved when the antenna is re-pointed. The restrictions on rotation or gimbaling around these mirrors makes achieving a wide field of regard difficult, because the antenna will rotate until the reflector hits the support structure 102 for the beam waveguide as illustrated in FIGS. 1A and 1B.
The restriction of no rotations between the parabolic mirrors 208 and 210 is due to the offset nature of the dual sets of paraboloids reflectors 204 and 206 in the beam waveguide 202 (FIGS. 2A and 2B). The configuration of the antenna system 200′ in FIG. 2B or similar rotations between reflectors 208 and 210 that produce geometries other than that of FIG. 2A are precluded. The paraboloids 204 serve to receive the feed radiation, beam or wave from the feed horn 216, and collimate the beam or wave so it can transmit loss-free from between paraboloid reflectors 208 and 210, and re-create a spherical wave or beam from the feed horn 216 at a point or focus 218 of the antenna assembly 220. The offset paraboloid set 204 generates a beam that has a coherent, planar phase front between paraboloid reflectors 208 and 210, but has an asymmetrical field distribution around an axis 222 between the paraboloid reflectors 208 and 210. If paraboloid reflector 208 has an identical geometry to paraboloid reflector 210 and is aligned therewith, the wave reflecting from paraboloid reflector 208 will re-create the spherical wave pattern from the feed horn 216 at the focal point 218 of the antenna assembly 220 because the offset-induced field distortions will cancel out. If the paraboloid reflectors 208 and 210 are not identical or are rotated as shown in FIG. 2B relative to FIG. 2A, the field pattern at focal point 218 will not be identical to the feed pattern from feed horn 216. Such distortions as a function of the rotation angle about the axis 222 between paraboloid reflectors 208 and 210 will cause a loss in antenna efficiency and may preclude auto-tracking of the beam of the antenna 220. The ability to auto-track the beam is a desired feature of high gain, narrow beam systems. Therefore, to avoid distortions and loss of antenna efficiency, no rotations between the paraboloids 208 and 210 may be permitted.